A backlight unit (BLU) which is a light emitting element widely used in a liquid crystal display (LCD) allows light emitted using a light source such as a cold cathode fluorescent lamp (CCFL), etc., to sequentially pass through a light guide plate, a diffusion sheet and a prism sheet to reach a liquid crystal panel.
The light guide plate transfers light such that light emitted from the light source is distributed onto the entire surface of the liquid crystal panel which is planar, the diffusion sheet makes light intensity uniform over the entire surface of the screen, and the prism sheet performs a light passage control function such that light rays that have passed through the diffusion sheet in a variety of directions are converted into an image within a range of a viewing angle adapted to be observed by a user. Also, a reflective plate is provided under the light guide plate such that light falling out of the light passage without being transferred to the liquid crystal panel is reflected again so as to be used, thereby increasing use efficiency of the light source.
In such a backlight unit, the prism sheet performs a light collection function, and controls the passage of light diffused in a variety of directions through the diffusion sheet so as to increase front luminance of the display, thus achieving a brighter and clearer image.
Recently, as displays are being made slimmer, a complex prism sheet is being developed, which is able to achieve both a diffusion function and a light collection function by introducing a light diffusion layer to the prism sheet. Furthermore, in order to reduce the cost of the display, the number of light sources is decreased in the backlight unit.
However, because the output of individual light sources should be increased to decrease the number of light sources in the backlight unit, variations (a portion distant away from a portion adjacent to the light source in the complex prism sheet) of heat transferred to the complex prism sheet should increase due to increased output of the light source. The variations of heat cause a difference in shrinkage and expansion at different positions of the complex prism sheet, and thus the complex prism sheet may wrinkle or curl due to the difference in shrinkage and expansion at different positions of the complex prism sheet.
When the complex prism sheet wrinkles or curls in this way, uniformity of light incident on the liquid crystal panel may decrease, undesirably deteriorating the quality of an image. That is, curling or wrinkling of the complex prism sheet expanded or shrunken due to variations of heat generated from the light source makes it difficult to efficiently introduce light to the liquid crystal panel from the light source. Thus, the development of techniques for solving this problem is required.